1. Field of Invention
This invention relates generally to devices comprising ion insertion materials and methods for manufacturing the same, and more particularly to methods of protecting ion insertion or intercalation materials in electrochemical cells, such as lithium ion batteries or electrochromic devices, to improve durability of such materials.
2. Description of the State of the Art
Electrochemical cells find utility in numerous devices such as lithium rechargeable batteries and electrochromic devices. Small-sized lithium rechargeable (secondary) batteries have been widely used as a power sources for portable electronic equipment in the fields of office automation equipment, household electronic equipment, communication equipment and the like. Electrochromic devices are highly beneficial in a variety of practical applications where light modulation is desirable. These include, for example, alphanumeric displays for clocks, watches, computer monitors, outdoor advertisement and announcement boards, and other types of displays. In addition, an important application for the electrochromic devices of the present invention is light modulation in, for example, mirrors of variable reflectance (as are used in some automotive rearview mirrors), sunglasses, automotive windshields, sunroofs, and building windows. Both rechargeable lithium batteries and electrochromic devices operate on the principle of an electrochemical cell (also referred to as a galvanic cell). An electrochemical cell is a composite structure containing a negative electrode (the cathode), a positive electrode (the anode) and an ion-conducting electrolyte interposed therebetween.
A conventional lithium rechargeable battery has a negative electrode (the cathode) comprising an active material which releases lithium ions when discharging, and intercalates or absorbs lithium ions when the battery is being charged. The negative active materials commonly utilized in lithium ion batteries include niobium pentoxide, carbon, and similar materials capable of intercalating lithium ions. The positive electrode (the anode) of a conventional lithium ion battery contains a substance capable of reacting chemically or interstitially with lithium ions, such as transition metal oxides, including vanadium oxides, cobalt oxides, iron oxides, manganese oxide and the like. In general, the positive active material comprised by the positive electrode will react with lithium ions in the discharging step of the battery, and release lithium ions in the charging step of the battery. Since both the anode and cathode materials of lithium ion batteries can intercalate lithium ions, the anode and cathode materials are often referred to as xe2x80x9cion insertion materialsxe2x80x9d or xe2x80x9cintercalation materials.xe2x80x9d The external faces of the anode and cathode lithium ion batteries are usually equipped with some structure or component to collect the charge generated by the battery during discharge and to permit connection to an external power source during recharging. Conventional lithium ion batteries usually comprise a non-aqueous liquid or a solid polymer electrolyte, which has dissolved lithium salt that is capable of dissociating to lithium ion(s) and an anions, such as for example lithium perchlorate, lithium borohexafluoride, and other lithium salts that are soluble in the electrolyte utilized. During discharge, lithium ions from the anode pass through the liquid electrolyte to the electrochemically active material of the cathode, where the ions are taken up or absorbed with the simultaneous release of electrical energy. During charging, the flow of ions is reversed so that lithium ions pass from the electrochemically active cathode material through the electrolyte and are plated back onto the anode.
Another example of an electrochemical cell is an electrochromic device, such as those used on electrochromic windows. Conventional electrochromic windows comprise multi-layered devices, similar to a lithium secondary battery, comprising a pair of transparent electrodes sandwiched between two transparent substrates. A pair of ion-insertion materials, referred to as the electrochromic layer and an ion storage layer, are sandwiched between the pair of electrodes. The electrochromic layer of an electrochromic device is an electrochromic ion insertion material, which reversibly changes its color by the injection or extraction of ions as a result of an application of an electric potential. This reversible color change in a material caused by an applied electric field or current is known as xe2x80x9celectrochromism.xe2x80x9d The ion storage layer of an electrochromic device is an ion insertion material, which may or may not have electrochromic properties. An ion-conducting material (also known as an electrolyte layer) is disposed between the electrochromic layer and the ion storage layer. Positive ions are induced by the voltage to move through the ion conducting material, i.e., electrolyte, in the direction from the ion storage layer and toward the electrochromic layer. Upon application of a voltage across the electrochromic device, electrons flow through an external circuit in a direction from the electrode adjacent the ion storage layer to the electrode adjacent the electrochromic layer. Simultaneously, a resulting current is conducted by ions, such as lithium ions (Li+) or hydrogen ions (H+). The positive ions are induced by the voltage to move through the ion conducting layer in the direction from the ion storage layer and toward the electrochromic layer.
An example of an electrochromic material used in an electrochromic device is a tungsten oxide (WO3) film. To color the W03 film, a battery is connected between the pair of transparent conductive electrodes. When a negative voltage is applied to one of the electrodes (the negative electrode), electrons from the negative electrode and lithium ions from the lithium electrolyte are injected simultaneously into the WO3 film. This ion injection process continues until the colorless WO3 is converted into the blue-colored LixWO3. To bleach the blue-colored LixWO3 film, the polarity is reversed so that the electrons and lithium ions are depleted from the LixWO3 film. Current flows until the entire film is restored to its original WO3 (colorless) state. Thus, it is convenient to think of the coloring and bleaching process of an electrochromic device as the charging and discharging of a battery. Typically, for maximum efficiency, electrochromic devices include an electrochromic layer comprising an electrochromic material and an ion storage layer comprising a xe2x80x9ccomplementaryxe2x80x9d electrochromic material, i.e., an electrochromic layer that becomes colored upon positive ion insertion and an ion storage material that becomes colored upon removal of positive ions. As a result of this type of complementary system, the electrochromic and ion storage layers change color simultaneously as a result of an applied voltage to produces a more highly colored (darker) state.
Electrochemical devices such as lithium secondary batteries and electrochromic devices can use either a solid, liquid, or polymer gel-type electrolyte as the ion conducting layer, and therefore are referred to as either solid-state, liquid or polymer gel (also known as gel-type) devices, respectively. The ion conducting layer must possess high ionic conductivity (i.e., conducts positive ions such as Li+ or H+) and low electronic conductivity (does not conduct electrons).
Solid-state electrochemical devices have solid thin-film electrolytes made of so-called fast-ion conductor materials, in which either lithium or hydrogen ions diffuse readily. Examples of such fast-ion conductor materials include Li3N, Li2NH, Li1xe2x88x92xMxTi2xe2x88x92x(PO4)3, and LiAlF4. During the manufacture of solid-state electrochemical devices, the solid electrolyte layer (which is disposed between the cathode and the anode) is deposited in a manner which often results unavoidably in the formation of xe2x80x9cpinholesxe2x80x9d. Pinholes are defects in the solid electrolyte layer which act as electron xe2x80x9cchannelsxe2x80x9d between the cathode and the anode, such as the electrochromic layer and the ion storage layer in an electrochromic device. Consequently, in an electrochromic device, if a pinhole is present in the solid electrolyte layer, electrons will flow from the electrochromic layer, through the pinhole in the solid electrolyte layer, and back to the ion storage layer. Under this condition, known as xe2x80x9cshortingxe2x80x9d, electrons do not remain in the electrochromic layer during applied voltage; therefore, the electrochromic device cannot remain colored. Due to the inherent pinhole defects in the manufacture of solid state electrochromic devices, it is difficult to scale up these devices for larger applications, such as for electrochromic windows.
Liquid or gel-type electrochemical devices were developed to alleviate the xe2x80x9cshortingxe2x80x9d problems associated with solid state electrochemical devices. Liquid or gel-type electrochemical devices have a liquid or gel material as the ion conducting layer, which is typically formed by sandwiching the liquid or gel-type ion conducting material between the cathode and the anode after the electrochemical device has been assembled. Consequently, liquid electrochemical devices do not suffer the drawback of pinholes as in solid-state devices. Therefore, they are easier to scale up than the solid state devices. However, liquid or gel-type electrochemical devices are often less durable than solid state devices, possibly due to degradation of the ion storage layer and the electrochromic layer by the liquid electrolyte. As the electrochromic and ion storage layers degrade, it becomes necessary to apply increasing amounts of voltage or current to the device to achieve the same degree of color intensity.
A need therefore exists for a liquid or gel-type electrochemical device that has increased durability and wherein the ion insertion materials do not suffer from the degradative effects of being in contact with the liquid or polymer gel electrolyte as in conventional liquid or gel-type electrochemical devices.
Accordingly, objects, features and advantages of the present invention are to provide an improved liquid or gel-type electrochemical cell based, for example, on lithium, which maintains its integrity over a prolonged life-cycle as compared to conventional liquid or gel-type electrochemical cells, and to provide a protective, solid ion conducting layer between the ion insertion material(s) and the liquid or gel-type electrolyte, wherein the protective layers prevent degradation of the ion insertion materials. The protective layers are characterized by an ability to conduct positively charged ions but are poor electronic conductors. The protective layers are of a sufficient thickness to restrict penetration of the liquid electrolyte layer and consequently reduce or prevent degradation of the ion insertion layer(s).
Accordingly, it is a general object of this invention to provide for a method of protecting an ion insertion material having a surface which faces a liquid or gel-type ion conducting material.
A more specific object of this invention is to provide a liquid or gel-type electrochemical device having increased durability.
Another specific object of the present invention is to provide a liquid or gel-type electrochemical device having improved cycling lifetime.
Another specific object of the present invention is to provide a liquid or gel-type electrochemical device having improved durability comprising a solid ion conducting layer disposed between a ion insertion layer and a liquid or gel-type ion conducting layer.
Another specific object of the present invention is to provide a liquid or gel-type electrochromic device which is able to maintain a substantially constant color intensity over time with repeated application of an electric current.
Another specific object of the present invention is to provide a liquid or gel-type electrochromic device having improved durability comprising a solid ion conducting layer disposed between an ion-insertion layer and a liquid or gel-type ion conducting layer.
Another specific object of the present invention is to provide a method of manufacturing a liquid or gel-type electrochemical cell having increased durability.
Additional objects, advantages and novel features of this invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following specification or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities, combinations, and methods particularly pointed out in the appended claims.
To achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described therein, an electrochemical cell of this invention comprises a pair of substrates, a pair of electrodes sandwiched between the pair of substrates, a pair of ion-insertion layers sandwiched between the pair of electrodes, a pair of solid ion conducting layers sandwiched between the ion insertion layers, and a liquid or gel-type ion conducting material disposed between the solid ion conducting layers.